Storage devices of various types are utilized for storing information such as in computer systems. Conventional computer systems include storage devices such as disk drives for storing information managed by an operating system file system. With decreasing costs of storage space, an increasing amount of data is stored on individual disk drives. However, in case of disk drive failure, important data can be lost. To alleviate this problem, some fault-tolerant storage devices utilize an array of redundant disk drives (RAID).
In typical data storage systems including storage devices such as primary disk drives, the data stored on the primary storage devices is backed-up to secondary storage devices such as tape, from time to time. However, any change to the data on the primary storage devices before the next back-up, can be lost if one or more of the primary storage devices fail.
True data protection can be achieved by keeping a log of all writes to a storage device, on a data block level. In one example, a user data set and a write log are maintained, wherein the data set has been completely backed up and thereafter a log of all writes is maintained. The backed-up data set and the write log allows returning to the state of the data set before the current state of the data set, by restoring the backed-up (baseline) data set and then executing all writes from that log up until that time.
To protect the log file itself, RAID configured disk arrays provide protection against data loss by protecting a single disk drive failure. Protecting the log file stream using RAID has been achieved by either a RAID mirror (known as RAID-1) shown by example in FIG. 1, or a RAID stripe (known as RAID-5) shown by example in FIG. 2. In the RAID mirror 10 including several disk drives 12, two disk drives store the data of one independent disk drive. In the RAID stripe 14, n+1 disk drives 12 are required to store the data of n independent disk drives (e.g., in FIG. 2, a stripe of five disk drives stores the data of four independent disk drives). The example RAID mirror 10 in FIG. 1 includes an array of eight disk drives 12 (e.g., drive0–drive7), wherein each disk drive 12 has e.g. 100 GB capacity . In each disk drive 12, half the capacity is used for user data, and another half for mirror data. As such, user data capacity of the disk array 10 is 400 GB and the other 400 GB is used for mirror data. In this example mirror configuration, drivel protects drive0 data (M0), drive2 protects drivel data (M1), etc. If drive0 fails, then the data M0 in drivel can be used to recreate data M0 in drive0, and the data M7 in drive7 can be used to crate data M7 of drive0. As such, no data is lost in case of a single disk drive failure.
Referring back to FIG. 2, a RAID stripe configuration effectively groups capacity from all but one of the disk drives in the disk array 14 and writes the parity (XOR) of that capacity on the remaining disk drive (or across multiple drives as shown). In the example FIG. 2, the disk array 14 includes five disk drives 12 (e.g., drive0–drive4) each disk drive 12 having e.g. 100 GB capacity, divided into 5 sections. The blocks S0–S3 in the top portions of drive0–drive3 are for user data, and a block of drive4 is for parity data (i.e., XOR of S0–S3). In this example, the RAID stripe capacity is 400 GB for user data and 100 GB for parity data. The parity area is distributed among the disk drives 12 as shown. Spreading the parity data across the disk drives 12 allows spreading the task of reading the parity data over several disk drives as opposed to just one disk drive. Writing on a disk drive in a stripe configuration requires that the disk drive holding parity be read, a new parity calculated and the new parity written over the old parity. This requires a disk revolution and increases the write latency. The increased write latency decreases the throughput of the storage device 14.
On the other hand, the RAID mirror configuration (“mirror”) allows writing the log file stream to disk faster than the RAID stripe configuration (“stripe”). A mirror is faster than a stripe since in the mirror, each write activity is independent of other write activities, in that the same block can be written to the mirroring disk drives at the same time. However, a mirror configuration requires that the capacity to be protected be matched on another disk drive. This is costly as the capacity to be protected must be duplicated, requiring double the number of disk drives. A stripe reduces such capacity to 1/n where n is the number of disk drives in the disk drive array. As such, protecting data with parity across multiple disk drives makes a stripe slower than a mirror, but more cost effective.
There is, therefore, a need for a method and system of providing cost effective data protection with improved data read/write performance than a conventional RAID system. There is also a need for such a system to provide the capability of returning to a desired previous data state.